To ensure a similar spectral environment, each S segment for FRET includes additional Ala residues at the N- and two Ala residues at the C- terminals. The acceptor dye (5-(Dimethylamino)naphthalene-1-sulfonyl, Dansyl) is attached to the N-terminal while the donor dye (Naphthyl) is attached to the C-terminal. For P segments, we use Trp as a FRET donor and 7-acetamido-4-coumarincarboxylic acid (Coumarin), coupled to cysteine residue, as a FRET acceptor. The dye pairs of both S and P segments have a Förster distance, \(R_0\) , of 24 Å[1]}, [2]}. As a control, one labeled S segment is also measured using SAXS and compared with unlabeled (but with equal sequence) S segments (Fig. REF ). The labels increase the \(R_g\)  by about \(\sim \)  1 Å.
